Flexible, full color, bistable displays continue to be of keen interest to the display industry. The ability to create a flexible bistable display at low cost and with various images has the potential for numerous applications, including: point of sale advertising, hand held device decoration, indicators, fashion statements, writing tablets, electronic labels, among others. Previously Kent Displays Inc. has presented electronic skin (eSkin) and writing tablet displays, which allow for the respective applications of switchable color for consumer device cases and pressure sensitive writing tablets.
The Reflex display technology, based on cholesteric liquid crystals (ChLC), offers reflective, flexible, full-color, bistable displays for numerous applications. The ChLC material is a natural reflector since its inherent chiral structure leads to a Bragg-type reflection of the incident light. There are two bistable textures; the planar texture or reflective state and the focal conic texture which is slightly scattering. Since there is an ink coating on the immediate back side of the display, the focal conic texture being only slightly light scattering, transmits undiffused light to the ink coating, which is then absorbed, making this texture appear as the display's dark state. Because the ink coating is near the cholesteric material, the focal conic texture is essentially or substantially transparent. The Reflex display technology has seen recent success with the Boogie Board® writing tablet, a dedicated electronic writing device as described in U.S. Pat. No. 6,104,448. The writing tablet is a single layer device that allows the user to write and draw images with analog resolution in the same way that they would with pen on paper. In addition to the writing tablet, another Reflex display technology is the eSkin, which offers truly real time switchable color for consumer device case covers, for example, including the added advantage of transforming the device case into a discrete indicator, while creating the ultimate in device personalization. The eSkin includes multiple display layers to create multicolor reflectance.
Flexible cholesteric writing tablet displays such as those made using the processes described in U.S. Pat. No. 7,351,506, are normally coated with a continuous black absorbing layer to provide contrast of the written image. The black coating is applied on the substrate opposite the one through which one would view the display image (i.e., to the substrate most distal from the viewer of the display). This light absorbing layer is fixed and opaque. In some cases, as disclosed in Published U.S. Patent application Pub. No. 2010/0245221, entitled “Display with Overlayed Electronic Skin,” this back painting is skipped altogether. The continuous light absorbing layer allows for a uniform display with one homogenous image. In addition, different color backpaints have also been discussed in the literature, in U.S. Pat. No. 5,493,430. Typical displays show images by creating numerous pixels inside the display where each image is created by switching each pixel to a different color.
This Boogie Board® tablet offers a considerable improvement over previous tablet technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrodes in the tablet. In a cholesteric liquid crystal writing tablet, the liquid crystal is sandwiched between two substrates that are spaced to a particular gap. The upper substrate is flexible and the bottom substrate is painted with a fixed opaque light absorbing dark background. Within the gap is a bistable cholesteric liquid crystal which can exhibit two textures, an essentially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The tablet is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the essentially transparent texture, with a low haze and transparency greater than any other display state. When one presses on the top substrate with a point stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from essentially transparent to a brilliant reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to transparent conducting electrodes on the inner surface of the substrates that drive the cholesteric liquid crystal from its color reflective state back to its essentially transparent state.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448, which is incorporated herein by reference. Polymer dispersions can be used to control the pressure sensitivity and resolution of the image as described in U.S. Patent Application Publication No. 2009/0033811, which is incorporated herein by reference. Other modes of operation and a tablet for multiple color images are described in this patent application publication and a means for select erase is disclosed in U.S. Patent Application Publication No. 2009/0096942, which is incorporated herein by reference and is applicable to the displays of the present disclosure.
The standard mode of operation for the Boogie Board® is termed Mode B where the ChLC is initialized to the focal conic texture and when pressure is applied locally to the display via a pointed stylus that region of the display flows to the planar texture creating a bright written line on a dark background. The other mode of operation is termed Mode A and in this mode the ChLC is initialized to the planar texture and when pressure is applied locally to the display via a pointed sylus that region of the display flows to the focal conic texture or some greyscale creating a dark line on a bright background. In Mode A a continuous AC or series of pulses is applied while the display is addressed with a pointed stylus.